A. Technical Field
The present invention relates to a continuous production process which is a process for continuous production of a water-absorbent resin product having high properties and, specifically, the invention relates to the continuous production process by which the water-absorbent resin product having the high properties can be obtained easily and inexpensively with stable constant quality. In addition, the present invention relates to a water-absorbent resin product having high properties and being stable in quality.
B. Background Art
In recent years, water-absorbent resins are widely used for sanitary materials such as disposable diapers, sanitary napkins, and so-called incontinent pads as their component materials for the purpose of causing the water-absorbent resins to absorb body fluids. Known examples of the- water-absorbent resins include: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; and crosslinked polymers of cationic monomers.
Such water-absorbent resins are required to have properties excellent in such as: liquid absorption quantity and water absorption rate when contacting with aqueous liquids (e.g. body fluids); gel strength; gel liquid permeability; and suction force to suck up water from aqueous-liquid-containing base materials. Furthermore, in recent years, water-absorbent resin powders having very narrow particle diameter distributions and water-absorbent resin powders of which the absorption capacities are high and the water-extractable-component contents are low are demanded, and it is essentially being demanded that such as absorption capacity under load and liquid permeability under load should be high.
Many parameters and measurement methods which define the properties of these water-absorbent resins are disclosed in such as the below-mentioned patent documents.
For example, water-absorbent resins excellent in the gel strength, the extractable component content, and the water absorption capacity are proposed (e.g. refer to Reissued U.S. Pat. No. Re 32649 below). In addition, water-absorbent resins excellent in the liquid permeability without load, the water absorption rate, and the water absorption capacity are proposed (e.g. refer to GB 2267094 below). Furthermore, arts in which specific particle diameter distributions are defined are also proposed (e.g. refer to U.S. Pat. No. 6,087,002 below, and besides, such as U.S. Pat. Nos. 5,051,259, 5,419,956, and EP 0629441).
In addition, many water-absorbent resins excellent in water absorption capacities under various loads and their measurement methods are also proposed, and water-absorbent resins which are defined by their water absorption capacities under load alone or in their combinations with other properties are proposed (e.g. refer to U.S. Pat. No. 6,297,335 below, and besides, such as EP 0707603, EP 0712659, EP 1029886, U.S. Pat. Nos. 5,462,972, 5,453,323, 5,797,893, 6,127,454, 6,184,433, and reissued U.S. Pat. No. Re 37021).
In addition, water-absorbent resins excellent in the impact resistance are proposed (e.g. refer to U.S. Pat. No. 6,414,214 below, and besides, such as U.S. Pat. No. 5,140,076). Furthermore, water-absorbent resins of which the dust contents are defined are proposed (e.g. refer to U.S. Pat. No. 5,994,440 below), and water-absorbent resins of little color are proposed (e.g. refer to U.S. Pat. No. 6,444,744 below). Water-absorbent resins excellent in the gel durability against such as aqueous L-ascorbic acid solution as to the urine resistance and excellent in the water absorbency are proposed (e.g. refer to U.S. Pat. No. 6,194,531 below, and besides, such as EP 0940148). Water-absorbent resins excellent in the gas permeability are proposed (e.g. refer to EP 1153656 below). In addition, water-absorbent resins of which the residual monomer contents are low are proposed (e.g. refer to EP 0605215 below).
Furthermore, it is proposed that water-absorbent resins having specific properties are favorable for water-absorbent articles (diapers) having specific properties, specific constitutions, or specific polymer concentrations (e.g. refer to U.S. Pat. No. 6,150,582 below, and besides, such as U.S. Pat. Nos. 5,147,343, 5,149,335, EP 0532002, U.S. Pat. Nos. 5,601,452, 5,562,646, 5,669,894, and international publication WO 02/053198).
In recent years, as the production of water-absorbent resins increases, the steps (e.g. polymerization, surface treatment) for their production are increasingly made continuous (e.g. refer to EP 0937739 below). In the case where water-absorbent resins are continuously produced, all the produced water-absorbent resins are desired to have the same high properties. However, actually, there may occur dispersions in properties between such as lots of the continuously produced water-absorbent resins, because water-absorbent resins having much more or much less than aimed properties may mingle as impure components (e.g. refer to: International Publication Pamphlet No. 2002/10451; and JP-A-349625/1999). In addition, as demanded properties become higher, or as the production process made continuous becomes longer, or as the production scale becomes larger, slight dispersions in properties in the steps give great dispersions to properties of final water-absorbent resin products. In addition, the below-mentioned surface-crosslinking, particularly, dehydration esterification, also exercises influence on dispersions in properties between final water-absorbent resin products. In addition, if attempts are made to suppress such dispersions in properties, then such attempts have hitherto often involved the deterioration in the productivity. Furthermore, various kinds of water-absorbent resins which meet users' requirements are demanded. As a result, there have been problems such that the production process becomes complicated.
In recent years, in sanitary materials for absorption of excrement, urine, or blood (another name: absorbent articles) which are represented by disposable diapers, there is increasing the concentration of the water-absorbent resin (amount of the water-absorbent resin as used). Therefore, there have been problems such that it takes a long time to stabilize the properties when the product number (type) is changed, and further that slight dispersions in properties of such water-absorbent resin products exercise great influence on final water-absorbent articles.